As electronic devices become smaller and operate at higher speeds, energy emitted in the form of heat increases dramatically. A popular practice in the industry is to use thermal grease, or grease-like materials, alone or on a carrier in such devices to transfer the excess heat dissipated across physical interfaces. Most common types of thermal interface materials are thermal greases, phase change materials, and elastomer tapes. Thermal greases or phase change materials have lower thermal resistance than elastomer tape because of the ability to be spread in very thin layers and provide intimate contact between adjacent surfaces. Typical thermal impedance values range between 0.6-1.6xc2x0 C. cm2/w.
A serious drawback of thermal grease is that thermal performance deteriorates significantly after thermal cycling, such as from xe2x88x9265xc2x0 C. to 150xc2x0 C., or after power cycling when used in VLSI chips. It has been also found that the performance of these materials deteriorates when large deviations from surface planarity causes gaps to form between the mating surfaces in the electronic devices or when large gaps between mating surfaces are present for other reasons, such as manufacturing tolerances, etc. When the heat transferability of these materials breaks down, the performance of the electronic device in which they are used is adversely affected. The present invention provides a thermal interface material that is particularly suitable for use as an interface material in electronic devices.
In accordance with the invention there is provided a compliant and crosslinkable material which comprises a silicone resin mixture, such as a mixture of vinyl silicone, vinyl Q resin, hydride functional siloxane and platinum-vinylsiloxane, a wetting enhancer and at least one thermally conductive filler. The compliant thermally conductive material has the capability of enhancing heat dissipation in high power semiconductor devices and maintains stable thermal performance. It is not subject to interfacial delamination or phase separation during thermal-mechanical stresses or fluctuating power cycling of the electronic devices in which it is used.
The compliant and crosslinkable thermal interface material may be formulated by mixing the components together to produce a paste which may be applied by dispensing methods to any particular surface and cured at room temperature or elevated temperature. It can be also formulated as a highly complaint, cured, tacky elastomeric film or sheet for other interface applications where it can be preapplied, for example on heat sinks, or in any other interface situations.
The filler to be incorporated advantageously comprises at least one thermally conductive filler, such as silver, copper, aluminum, and alloys thereof; boron nitride, aluminum nitride, silver coated copper, silver coated aluminum and carbon fiber. It may be also additionally useful to incorporate antioxidants to reduce oxidation of the resins, wetability enhancing agents to promote wetting of surfaces, curing accelerators, such as would allow curing at room temperature, viscosity reducing agents to enhance dispersability and crosslinking aids. It is also often desirable to include substantially spherical particles of filler to limit the compressibility of the compliant material in interface applications, i.e. to limit or control the thickness.
It has been also found that thermal conductivity of resin systems, such as a combination of filler and the combined silicone resin mixture discussed above, can be especially improved by incorporating carbon micro fibers, with other fillers, into the system.